The island of Fiore is a somewhat special island in the Grand Line. The Marines and the World Government are allied with the rulers of the island, but they do not technically have any authority over island affairs.
Most people on the island have never heard of a Devil Fruit, but Fiore has something different. Magic. A small portion of the population can use magic and are wizards. There are many different guilds.
Because of the dangerous waters around the island, and the powerful wizards defending the country, it is very rare for anybody to come to visit. Only a crew with a skilled navigator and a reckless, some might even say stupid, captain would dare to travel here.